I did not know about Oleg's posting, as I originally said, I based my implementation on
a paper by Conor McBride. Oleg is addressing the question of type safe casting, rather
than generic storage, so his code is a bit different. Infact his class:
> class TypeSeq t s where
> type_index:: t -> s -> Int
> fetch:: t -> s -> t
> alter:: t -> s -> s
>
> instance (PList Cons t r) => TypeSeq t (Cons t r) where
> type_index _ _ = 0
> fetch _ (Cons v _) = v
> alter newv (Cons v r) = Cons newv r
>
> instance (PList Cons t' r', TypeSeq t r') => TypeSeq t (Cons t' r') where
> type_index v s = 1 + (type_index v $ cdr s)
> fetch v s = fetch v $ cdr s
> alter newv (Cons v' r') = Cons v' $ alter newv r'
This stores unique types in a list that can be indexed by their types. Actually last
night (before I read this code) I came up with something similar:
data MNil = MNil deriving (Show,Data,Typeable)
data MCons l a r = MCons l a r deriving (Show,Data,Typeable)
class MLookup l r a | l r -> a where
mLookup :: r -> l -> a
instance MLookup l (MCons l a r) a where
mLookup (MCons _ x _) _ = x
instance MLookup l r b => MLookup l (MCons m a r) b where
mLookup (MCons _ _ xs) l = mLookup xs l
This is indexed by a unique type, but stores a second independant
type. The allows a kind of static finite map, which is pretty cool!
Here's an example:
data TmId = TmId
data TmVal = TmVal
data TmFloat = TmFloat
data TmName = TmName
testMap :: MCons TmId Int
(MCons TmVal String
(MCons TmFloat Float
(MCons TmName String
MNil)))
testMap = MCons TmId 1
$ MCons TmVal "Hello"
$ MCons TmFloat 1.2
$ MCons TmName "World"
MNil
main :: IO ()
main = do
putStrLn $ show $ testMap `mLookup` TmId
putStrLn $ show $ testMap `mLookup` TmVal
putStrLn $ show $ testMap `mLookup` TmFloat
putStrLn $ show $ testMap `mLookup` TmName
Index types don't need to be unique, the first match from the
head of the list will be returned. No match will result in a
compile time error.
Regards,
Keean Schupke.
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